December 2004

Image of 'Frogs, Flies, and Dandelions' by Menno Schilthuizen

December's book of the month is Menno Schilthuizen's Frogs, Flies, and Dandelions: Speciation - The Evolution of New Species. This book has been out for a number of years (my copy is from 2001). Menno Schilthuizen is a professor of biology at the University of Malaysia Sabah in Malaysia, and this book is an attempt to bring to light the formation of new species. Charles Darwin wrote on the formation of species over a century ago, yet despite this, there is still not much common understanding of just how species form. This book was developed for just that purpose. Let's take a look at what is inside.

Chapter One: Sorting out Life - What are Species Anyway?

Since this is a book about how species can form, it's important to start out with detailing what a species is. Traditionally, species have generally been thought of as groups of individuals which only breed among themselves. In a (very) general sense, this is pretty much the case, but as Schilthuizen points out repeatedly, there is no set definition of species, something he calls (as others have) The Species Problem. So, what do we use to identify species?

Schilthuizen walks us through the development of species in this first chapter, and with each advance, he introduces a problem each new definition encounters. Early on, taxonomists like Linnaeus used appearance and morphology to distinguish species, but when we look to animals such as horses and dogs (as Schilthuizen points out), we can see that it fails to help clarify relationships. Another more popular idea, the Biological Species Concept, or BSC, is the definition many people use to describe species, and it is roughly similar to the definition I gave previously. But, as Schilthuizen points out, there are problems here, too, in the case of hybrids. Schilthuizen finishes this chapter developing the concept of species which is useful for the remainder of the book.

Chapter Two: An Isolated Case? Geographical Speciation

One of the most basic ways in which species can form is with geographic isolation. This was a concept that Darwin himself didn't fully come to realize, but since his time, biologist's have studied extensively. The concept here is simple, really. Take a population of organisms, and then remove a sampling of that population and place it somewhere that it can not interact with the larger group. Let this sample group remain isolated for some generations, and then come back to record the characteristics. They will most likely be different than those of the original population. This is geographic isolation in a nutshell. Schilthuizen uses examples like cave beetles, gulls, and African cichlids to make the point about geographic isolation, and he does it very clearly. But, it isn't the only way species can form, as we will see in this next chapter.

Chapter Three: Tight Spots: The Magic of Small Populations

In tandem with the concept of geographic isolation is the concept of bottleneck populations, also known as the founder effect. Menno Schilthuizen seems uncomfortable with this idea, and spends this chapter discussing what people had thought were examples of bottleneck populations, and points out how they were not. He discusses several examples, and goes into good detail of why what was studied wasn't what happens in nature. As similar as bottlenecking is to geographic isolation, it seems strange why one would be supported (geographic isolation) and the other not (bottlenecks). What else can be driving speciation events?

Chapter Four: Seductive Theories: The Power of Sex

In this chapter, Schilthuizen introduces and explains the concepts of sexual selection, and the sexual arms race. Sexual selection was described by Darwin in his book On the Origin of Species, and even further in his book The Descent of Man and Selection in Relation to Sex, released a few years later. The majority of the examples used by scientists, and in this chapter, involve the intricate displays that birds put on. For example, the peacock has a very familiar display, that seems rather impractical when it comes to survival. It's large tail would be a hindrance to flight, and requires a lot of energy to grow. But, the drive for sexual reproduction seems to override any practicality, it would seem. Schilthuizen spends a lot of time developing these concepts, and uses it to bolster the notion of geographic isolation.

Chapter Five: Wham, Bam, Brand New Species: On the Instant Origin of Species

The traditional notion of speciation is that it takes some time for a new species to form. Schilthuizen points out in this chapter, that this isn't necessarily the case. Take polyploidy, for instance, which is an automatic doubling (or more) of the number of chromosomes in an organism. This is a phenomenon that is readily present in many plant groups, and whenever this happens, you've got an instant speciation event. Here's how it works. Species are generally thought of as reproductively isolated and genetically distinct populations. A plant, when it is reproducing, can from time to time, make a copy of itself whereby the number of chromosomes is doubled. This automatically renders it reproductively isolated from the parent plant, because the number of chromosomes won't match up. It is also genetically separate, because it was twice the number of genes. So, in effect, you've got an organism which is reproductively isolated and genetically distinct; in other words, a new species. Now, there are some critics to this notion of instant speciation, but regardless of their objections, it still holds that they are new species. Schilthuizen uses this chapter to describe instances of polyploidy both in plants and other living things to show this instant speciation.

Chapter Six: A Chronic Case of Rhagoletis: The Birth of an Evolutionary Heresy

One of the more difficult ideas in evolution is sympatric speciation, the type of speciation where a new species crops up right next to or within the population of its parent species. The convention holds that there has to be some kind of geographical isolation, and that sympatric speciation is, at best, a fairy tale. Schilthuizen discusses one of the best cases in support of this type of speciation, in a little fruit fly called Rhagoletis pomonella. This species of fly lives off of the hawthorne tree, but has recently developed a taste for apples. Apple trees were brought over from Europe by settlers, so are not native to the region R. pomonella lives in. It turns out that there is a population that is feeding and growing on apples, and has been called the Apple Maggot Fly. There is really nothing significant about the difference of these two variants, other than the length of their respective ovipositors and their choice of food. Of course, their choice of food fruits at different times of the year, so there is very little overlap for mating between the two species. This chapter describes in detail the struggle that a scientist named Guy Bush had when trying to suggest this as a case of sympatric speciation, which was considered a type of heresy among orthodox evolutionists. Bush showed that R. pomonella was a good case for sympatric speciation, but was it the only case, or were there more instances? This is the topic of the next chapter.

Chapter Seven: A Freak Show? Apple Maggots are not Alone

One case may be the exception to the rule, but how can a scientist show that an idea is more common than what the orthodoxy thinks? By presenting other instances of the very thing they suggest, of course. This chapter is filled with many other cases of sympatric speciation, from other insects to birds. Since the purpose of this chapter is to bolster the idea of sympatric speciation, there isn't much point in discussing it further, other than noting that there is indeed lots of support for it.

Chapter Eight: Ecotone - Speciation Prone? Kinks in the Environmental Spawn Species

So, what's an ecotone? It's an area of transition between different ecosystems. It can be big, or small. Schilthuizen talks about ecotones in this chapter because they provide an excellent place for sympatric speciation events. Some of the most dynamic speciation events have taken in place where there has been a transition zone created by humans, such as those found around mine pits. Mining can be very taxing on an ecosystem, raising toxins to more than lethal levels, but life, being persistent, still can find ways to inhabit these areas. Schilthuizen cites several examples of speciation that has occurred in ecotones, some man-made, some natural, and explains how these may be hotbeds for sympatric speciation.

Chapter Nine: Victoria's Blue Genes: Sex in Sympatry

This chapter takes a more in-depth look at the cichlids of Africa's Lake Victoria, many of which are not only new species, but have formed as new species within the last couple of centuries. This is speciation on a massive scale, and Schilthuizen takes the time to describe many of the species, some of which haven't been officially named yet.He points out that speciation in this instance can only have been sympatric, as there simply hasn't been enough time for such divergence that allopatric speciation would require. This is his last example on speciation events, and thus far he's made a pretty compelling case. So, with all of this background support, what does it all mean? This is the subject of the last chapter.

Chapter Ten: Mystery? What Mystery?

This chapter caps off what was an amazing adventure dealing with speciation and support for it. Schilthuizen now talks about life in the future, by mentioning species that seem to be in the process of forming new species, and describing how our impact will have effects on life. What does the future hold for speciation? It's hard to tell, so in the mean time, we should focus on what it's doing now.

Summary

This book is very interesting, and chock full of information. The main drawback, however, is that one gets an overwhelming feeling of this being somewhat of a personal attack on Ernst Mayr, or perhaps maybe its simply a challenge to the status quo. There are quite a few instances where it almost seems like Schilthuizen is proposing something as a kind of "in your face" response to mainstream evolutionary biologists. Once you look past this, though, there is a lot to glean from this book. For anyone interested in evolution, and speciation in particular, this is well worth the time to read.